Brake controllers typically use pressure control as part of antilock, traction and/or stability control systems. The accuracy of the pressure control is in part affected by the compliance within the brake control system, such as the compliance of each brake connected to the hydraulic system as controlled by the electronic controller. The compliance is also influenced by pressure versus volume relationship as manifested within the brake hydraulic system when actuating the mechanical connection by the hydraulic. Resultantly, variation in the pressure versus volume relationship, including other compliance effects, may exhibit minor changes such as in a pressure control estimation employed when using pressure control. Accordingly, it is desirable to improve the pressure control estimation by refining control or responsiveness.
Representatively, FIGS. 1 and 2 show a graph of pressure control for a rear left brake having a 0.3 mm cage clearance and a 0.7 mm cage clearance, respectively. Cage clearance is generally a measure of the linear difference between the outer diameter of the shoes and the inner diameter of the drum when there is no pressure control acting on the brakes. A pressure control estimate 11 is determined for a particular vehicle by modeling or by empirical testing and determination, whereby pressure control may be achieved by implementing a pressure control request 12 in a brake controller in order to achieve the desired pressure control estimate. When the brake controller implements the pressure control request 12, an actual pressure control of the brake is achieved as manifested by a pressure control measured response 13. It is recognized that the actual response 13 is determined by utilizing a pressure sensor in a test vehicle and may not be observable in a vehicle utilizing the invention to advantage because the invention necessarily eliminates the need for pressure transducers otherwise required for pressure control. The pressure control measured response 13, when compared to the desirable pressure control estimate 11 as shown in FIG. 1, results in a 26.5% mean pressure estimate error 14. The mean pressure estimate error is the average of the error between the actual pressure and the estimate pressure. The pressure control measured response 13, when compared to the desirable pressure control estimate 11 as shown in FIG. 2, results in a 267% mean pressure estimate error 15. Illustratively, only a 0.4 mm change in cage clearance causes an additional 241% error in the pressure control. Therefore, changes in cage clearance can affect braking performance and pressure control.
Therefore, there is a desire to provide refined pressure control with more accurate pressure estimation.